The invention relates to the manufacture of security tapes and security threads.
Security threads are used widely in security paper for banknotes and the like. Typically, they are incorporated into the banknote paper during manufacture of the paper although in some cases they could be adhered onto a surface of the paper after manufacture of the paper itself. Security tapes are used in a wide variety of applications such as tear tapes and the like.
Several proposals have been made in the past for increasing security of such tapes and threads. In one approach, an optically variable effect structure is directly embossed into a hard polymer such as polyester (PET) to create directly embossed threads. Although the resulting product is satisfactory, this approach requires heavy duty equipment and is generally carried out on widths less than 30 cm. This results in an expensive product and limits the output volume.
A typical example is shown in EP-A-0624688. In this case, holograms are deposited by transfer at predetermined positions along a ribbon and a transparent protective layer and optionally a varnish are then deposited on the ribbon. The resultant sheet is then cut into strips which are introduced into a sheet of paper during its manufacture. This is a relatively expensive process.
DE-A-3744650 describes directly embossing holograms into a varnish film provided with a metallic, reflective layer carried on a polymeric carrier directly. This approach provides an alternative to a transfer process and leaves the resultant hologram substantially unprotected and unless laminated is thus completely unsuitable for the manufacture of security tapes and security threads.
CA-A-2026542 (EP-A-0420261) again discloses a conventional transfer process in which a hologram is transferred from a carrier structure onto a substrate. Part of the carrier structure includes a protective layer which is exposed following the transfer operation to protect the holographic profile which would otherwise be exposed. Again, this is a relatively complex process, particularly since it requires the use of the protective layer. Furthermore, this protective layer is not self-supporting and would not provide the required durability for threads or tapes.
EP-A-0330733 (WO-A-89/08166) describes a security element in the form of a thread or strip including a plastic film having a non-metallic opaque coating with recesses in the form of characters and patterns through which colour contrasting materials can be viewed.
EP-A-0319157 describes a security thread having a continuous metal path along its length.
CA-A-2058268 (EP-A-0492407) has a disclosure very similar to CA 2026542.
In accordance with the present invention, a method of manufacturing a laminate structure for use in a security tape or thread comprises:
i) providing a first structure having a flexible carrier supporting an optically variable effect structure;
ii) adhering the first structure to a flexible substrate with the optically variable effect structure between the carrier and the substrate;
iii) removing the carrier;
iv) permanently bonding the carrier to the resulting laminate to generate a final laminate structure; and,
v) slitting the laminate structure from step iv) into at least two lengths.
The invention avoids the direct approach described above by using a multi-layer structure, the optically variable effect structure being adhered to the substrate following its creation. In this way, particularly in the case of embossing, the optically variable effect part of the structure is protected within the overall laminate. Furthermore, the use of a multi-layer structure allows a wide combination and selection of security print and other entities to be incorporated leading to considerable flexibility of design. The resulting laminate has high performance and durability and yet is much cheaper to manufacture and can be manufactured in wide widths of up to 1 m or more. Typically, the resulting laminate will withstand the high temperatures and corrosive conditions during incorporation into paper and have a brightness and durability which will last for the lifetime of the security document in which it is incorporated.
A further important advantage of the invention lies in the re-use of the carrier. Not only does this significantly reduce the cost of producing the security tape or thread since no additional material is required to provide the permanently bonded layer, but in addition allows very durable optically variable effect structures to be obtained. This is achieved by first providing that structure on a removable carrier where the bonding strength is much less than is obtained during the permanent bonding step iv), thus allowing large sheets to be embossed relatively easily, and secondly, by permanently bonding the carrier to the laminate after it has been provided with the optically variable effect structure.
The flexibility of this laminate approach will be appreciated from the following discussion illustrating the different types of materials which can be included within the laminate.
The optically variable effect generating structure provided in step i) can have any conventional form and can be used to generate holographic or diffraction grating effects and the like. Typically, the structure comprises a layer, such as a lacquer, which is modified so as to generate the optically variable effect as well as being optionally tinted. Thus, the layer could be embossed and optionally metallised or provided with a further layer of different refractive index as is well known in the art. In the case of metallisation, this may be a full or partial metallisation or a fully metallised layer could be partially demetallised to generate a pattern or microimages. In other cases, the optically variable effect generating structure can be formed by a single layer which has been modified internally to provide the optically variable effect, for example by modifying its refractive index.
In order to bond the optically variable effect generating structure to the substrate, a facing surface of the substrate or the layer on the carrier adjacent the substrate is coated with an adhesive. Suitable adhesives include hot melt, UV or daylight cured adhesives (or other thermoplastic or thermoset types). The adhesives could be clear or be tinted and thus include for example UV fluorescent or other additive materials.
The carrier provided in step i) preferably comprises a self-supporting plastics material such as polyester, for example ICI Melinex, or polypropylene. The overall structure is preferably a selected grade of holographic or optical transfer or hot stamp foil. To assist removal, a release layer may be provided between the carrier and the optically variable effect generating structure or the carrier could be treated, for example using corona discharge and the like, to provide a controlled bonding level.
Typically, step ii) comprises providing an adhesive between the optically variable effect structure and the substrate, although the substrate could, for example, have an inherent adhesive property which is used to bond it to the optically variable effect structure. In the case of an adhesive, this would typically be hot melt adhesive although other well known types of adhesive could also be used, for example UV curable.
The substrate can comprise any conventional material such as polyester or polypropylene or indeed any polymer film which has high clarity and is suitable for use as a thread or tape. Typically, the thickness of the substrate will be in the range 11-23 xcexcm. The substrate can be clear, tinted, back printed and the like.
It will be appreciated that following construction of the laminate (and after removal of the carrier (step iii))), the optically sensitive part of the structure is securely bonded onto the substrate with the remainder of the structure providing outward protection.
The carrier layer is permanently bonded using a suitable adhesive such as a hot melt, UV or daylight cured adhesive which again could be clear or be tinted. The carrier layer may itself be clear or tinted or printed with visible or invisible fluorescent inks and the like.
Preferably, particularly in the manufacture of security threads, following step iv) but prior to step v) the exposed surfaces of the laminate from step iv) are provided with coatings to enable the structure to be incorporated into paper. Typically, the coatings comprise a primer formed on the exposed surface and a top coat formed on the primer.
In other applications, the laminate from step iv) could be suitably coated with an adhesive on one side which is then used to allow the structure to be rolled onto the surface of a document in the form of a tape or thread.
In one preferred construction, all layers in the laminate (apart from any metallisation) are transparent to allow the optically variable effect structure to be seen from both sides. This is useful for some products where the laminate in the form of a thread or tape can be seen from both sides.
Typically, the optically variable effect structure will be in a continuous form which reduces any problems of registration.
In general, the composite structure can be used as a thread, tape or strip bonded to a security substrate before or after printing and finishing by use of cold or heat bonding adhesives.